Peavey Bandit (1981) loud hum

[JimmyTolson]

on the 3 47ohm resistors (the small brown ones with strips beside the heatsinks) in line with the output transitors, 2 show 0.00 on the ohm meter while one moves up and down in the numbers but slow. all are still mounted to the board tho. if this one is suspected being bad what would the part number be on mouser.i agree with Jazz p bass on the shorted output transistor but thinking the one little 47ohm one may have caused it.

[Jazz P Bass]

Generally speaking, resistors do not 'cause' failures.
The 47 ohm resistors that you are referring to will burn up if the output transistors go shorted.
Testing them in circuit can be misleading as other parallel paths exist.
The one that shows '0.00' is most probably pointing to a shorted transistor.
Labeling the parts that you want to talk about is a good idea.
You can use the part designator that is labeled on the schematic.
Can you please list the test equipment that you have available?
The transistors can be checked with a decent multimeter.
Preferably one with 'diode check' function.
If you do not know how to check a transistor, Google it.

[JimmyTolson]

Okay i changed the 3 driver transitors 5331 and 5332 + bought new output transistors to make sure i had the right ones and there is no change in hum or speaker standing out. i made the lightbulb limiter as suggested to hook between amp and electric and without the speaker hooked up the lightbulb glows real dim but when speaker is hooked up lightbulb glows bright.the 3 resistors that are beside the little heatsinks beside the driver transistors (2 on one and 1 on the other) all check out to 47ohms.

[Jazz P Bass]

Now it's time to take some voltage readings.
With the amp hooked through the limiter & the speaker disconnected.
Look at the schematic.
There are test voltages galore.
What you want to find is something way abnormal from the schematic voltages.
It may help if you keep in mind what this output section is doing.
It is a balancing act between the + & - rail voltages.
The output transistors should be seeing around .500 - .600 volts on there bases. In other words 'barely' turned on.
That will place the driver transistor bases at around 1.1 volts.
Start there.

[52 Bill]

Have we ever found out how much and what kind of voltage is on the output of this amp?

I'm really afraid that if Jimmy keeps throwing parts at this amp there will be far more to repair than what was originally wrong with it in the first place.

[Jag]

I POSITIVE he's going to be buying a new speaker soon!

As 52Bill said in the very first response to your question

Do not leave the amp on with the speaker connected, as it will cause damage to the speaker.

.

Seriously, DON'T connect the speaker until you have resolved the problem. If you want to know if there is still DC at the output, use a meter to check NOT the speaker!

[DrGonz78]

The DC on the speaker is the hum, so measure that w/ meter. Do not think that the meter will show something different than when you plug in the speaker to take a listen. Also, I was wondering if you had tested the rectifier diodes? In the original picture this diode looks funny to me... Probably just the shadow or something, but it looks chipped or something. I bet that is just the shadow, but I would still test those diodes. Also, test all your voltages foremost...

[JimmyTolson]

I looked at the rectifier diode you circled in the pic above and there is a raised line like crack on the side toward the edge of the board. the numbers are hard to read but it looks the same as the one below it and the 2 between the big blue capacitors. the 2 beside the white square thing are smaller with different numbers. ill post closer pics of it.( the meter pic is of what i have to check things with.)

[DrGonz78]

I have read a post of these diodes creating hum before and the guy had changed so many things before the rectifier diodes. To me it looks like there is maybe a crack in it and then someone put a dab of glue on it...? Not sure if that is what I am seeing here. Did you test the diode? Now if all the diodes have this hair line crack then that is how they will look, so make sure I am not leading astray. If it looks like more than hairline then I would probably be questioning that part more. I was just seeing something that was in the picture and that may(probably not) not be the problem here...

Put your multimeter on the diode checking feature and test it both ways with the amp off. One way will read I or OL and the other will read .500 or .700 or something around those types of readings. My feeling here is that they are probably fine but you should always check. If you get funny readings then sometimes you need to lift a leg on a diode to pull it out of circuit to get definite readings. The fact that the amp is passing the signal to the output make me think that these are probably working just fine.

Edit: Get on track w/ the light bulb limiter running the amp and get those output transistor voltage readings foremost.

[JimmyTolson]

i pulled one leg of the diode in question and checked it. one way does nothing on the meter as it stays at 1 and when i reverse the wires it shoots to 620. this is using the diode check on the meter in pic above (->l-) .

[DrGonz78]

So it looks good. Sorry to get you going in another direction, but that is good to know that the part measures correctly. How is it going w/ your voltage measurements? Let us know when you test those output transistors. Best regards

[JimmyTolson]

ive done everything to my scope on this and im having no luck at all.the nearest place to me is over a 2 hour drive one way that can even look at it. would be willing to u.p.s it to someone on here that knows about these amps to get it fixed. would be glad to pay plus pay for return shipping if anyone is interested.

[Jazz P Bass]

You still have not posted any voltage readings.
Basic stuff.
Like the power supply rails, the base voltages of the output transistors.
That would basically require a volt meter.
(unless the scope was set to read Vdc.)

[J M Fahey]

If you enter the CP5 in their search window, then select the resistance of the passive component parts, and find them on your windowsill.

Sorry Motumoyo, don't get you.
What are you trying to say?.
Thanks.

[DrGonz78]

In a galaxy far, far away... C3 PO and R2 D2 were in a search window trying to save us all from bots...